OK, you made me go dig in my book pile. The first one I came to was Andy Lennon's R/C MODEL AIRCRAFT DESIGN, PRACTICAL TECHNIQUES FOR BUILDING BETTER MODELS

He mentions the increase of max CL in almost every section but most sections deal with the special purpose models he was having fun designing. The general section on wing design has a piece on slotted flaps. I got no problem ignoring that. The next section is on plain flaps. Plain flaps are still flaps. And the ones he shows in a diagram are basically 25%C "rear of the wing" movable surfaces that are obviously hinged at .75C. He talks about their effect on symmetrical airfoils, particularly an E168. He says they add an additional CL of 0.62 at 60degree deflection.

I'd interpret that as saying, if you built a wing using an E168, if you made the rear of it into plain flaps, the airfoil that once gave you X amount of lift would now give you more when you used the flap part as a flap. And you can compute lift of the wing unflapped with the original CL and the flapped amount with the original CL plus 0.62.